Objective: To study the molecular basis for the generation of stable mutant phenotypes in heteroploid somatic mammalian cells in culture, and to analyze the role of somatic mutational processes in cellular transformation to malignancy. Approach: Mammalian cell lines with stable drug-resistance and enzyme-deficiency phenotypes are being studied with biochemical and immunochemical methods, in order to identify the genetic lesions which confer the variant phenotypes. Mouse cell mutants resistant to 8-azaguanine are used for genetic complementation tests in cell hybrids to isolate mutants which involve regulatory as well as structural gene mutations at the HPRT locus. Also, we have shown that malignant growth of animal cells in vivo in the athymic nude mice is correlated specifically with anchorage independent proliferation in vitro. This correlation has been extended to cells transformed with oncogenic viruses such as SV40 and murine sarcoma virus. We are now attempting to develop a model system for genetic analysis of malignant transformation of diploid cells, by combining the use of chemical mutagenesis for induction of specific point mutation products with the use of the nude mice for the assay of malignant growth potential in vivo.